The present invention relates to a master making device for a printer and, more particularly, to a master making device for a stencil printer and including a thermal head and a platen roller.
A digital thermal stencil printer is extensively used because of its simple printing system. This kind of printer includes a thermal head having a plurality of fine heating elements arranged in an array in the main scanning direction. While the head is pressed against a platen roller via a thermosensitive stencil, the heating elements are selectively energized by pulses. At the same time, the stencil is conveyed by the platen roller in the subscanning direction perpendicular to the main scanning direction. As a result, the stencil is perforated, or cut, by heat in accordance with image data. The perforated part of the stencil, i.e., a master is automatically conveyed to and wrapped around a porous cylindrical drum. Subsequently, a press roller or similar pressing means continuously presses a paper or similar recording medium against the master. Consequently, ink is transferred from the drum to the paper via the perforations of the master, forming an image on the paper.
A master making device is included in the printer in order to make the above master. Usually, a nip at least several times as great as the dimension of the heating element array, as measured in the subscanning direction, is formed between the thermal head and the platen roller, taking account of the scatters of the platen roller and other related parts in the subscanning direction and the scatter of the position of the heating element array as well as other scatters. The center of the heating element array in the subscanning direction is coincident with the center of the platen roller in the nip. Let this type of master making device be referred to as a first type of master making device.
Japanese Patent Laid-Open Publication No. 6-328653, for example, proposes a master making device in which the heating element array is deviated to the upstream side from the center of the platen roller in the subscanning direction within the nip. The above document teaches that with this configuration it is possible to set a required nipping length after perforation and therefor to produce a master free from creases ascribable to shrinkage even when the master is implemented by a stencil substantially consisting only of a thermoplastic resin film. This type of master making device will be referred to as a second type of master making device hereinafter.
A stencil for use in a thermal stencil printer has a laminate structure made up of an extremely thin film of polyester or similar thermoplastic resin and a porous base or support permeable to ink. The base is formed of synthetic fibers, Japanese paper or a combination thereof. There has recently been developed a stencil including a base entirely formed of fine synthetic fibers or formed of a mixture of natural fibers and fine synthetic fibers in order to improve image quality. This kind of master, or synthetic fiber base master as referred to hereinafter for distinction, is not as thin as the stencil substantially consisting only of a thermoplastic resin film (about 1 xcexcm to 8 xcexcm thick), but thinner than the traditional stencil (about 40 xcexcm to 50 xcexcm thick). Specifically, the synthetic fiber base master is about 10 xcexcm to 30 xcexcm thick and lower in rigidity or elasticity than the stencil whose base is formed of natural fibers.
Assume that the stencil whose base is formed of natural fibers has a coefficient of friction xcexc of 1, as measured on the ba se surface of the stencil. Then, the base surface of the synthetic fiber base master has a coefficient of friction xcexc of about 0.8 lower than 1. On the other hand, the smoothness of the film surface of the stencil depends on the diameter of fibers constituting the base. For example, as for the stencil whose base is formed of natural fibers, the fibers have a greater diameter than the synthetic fibers of the synthetic fiber base master and render the base surface irregular.
Because the film is adhered to such an irregular base surface, the smoothness of the film surface is lower than that of the film surface of the synthetic fiber base master whose fibers have a small and uniform diameter. The synthetic fiber base stencil is therefore higher in the smoothness of the film surface than the stencil whose base is formed of natural fibers.
The first and second types of master making devices described above each has the following problem left unsolved. Assume that the first type of master making device perforates the synthetic fiber base stencil thinner, less elastic and softer than the traditional stencil with the head, and conveys the perforated stencil or master with the platen roller. Then, the conveying force of the platen roller decreases because the surface of the base of the master being pressed by the platen roller has its coefficient of friction reduced and because the film surface of the master has its smoothness increased. As a result, the film of the stencil sticks to the surfaces of the heating elements of the head due to heat stored in the platen roller due to a master representative of a solid image having a substantial area. This causes the platen roller and master to slip on each other frequently and thereby reduces the master making length. Consequently, the conveyance of the master is deteriorated. This is also true with the second type master making device and presumably occurs, in greater or less degree, when use is made of a stencil including a film.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-156520, 9-71030 and 57-157771 (corresponding to Japanese Patent Publication No. 64-7589).
It is therefore an object of the present invention to provide a master making device for a printer capable of providing a master with an accurate length without regard to the kind of a stencil and insuring stable conveyance of the master.
A master making device of the present invention includes a thermal head having a plurality of heating elements arranged in an array in the main scanning direction. A platen roller forms a nip between it and the thermal head for pressing a stencil. The platen roller is rotatable for moving the stencil in the subscanning direction perpendicular to the main scanning direction. The position of the array of heating elements in the subscanning direction is deviated, within the nip, from the center of the platen roller to the downstream side in the subscanning direction to thereby reduce the length of the perforated portion of the stencil to be moved in the nip.